Check the target field in the Firefox desktop shortcut and make sure that you do not start Firefox with the -no-remote switch or have set the environment variable MOZ_NO_REMOTE=1 (Control Panel > System > Advanced > Environment variables).

The variable MOZ_NO_REMOTE is not set. I use only the default profile. I have had multiple in the past, but after reinstalling FF, only one.
But I have multiple Thunderbird profiles ... is there may be a depdendcy ?

isnt it strange, that it works from 1 external applikation if I klick a link. any other app with links came up with the same error as wehn I start FF directly. Something locket the execution either for FF start only or 1 App+FF if it is invoked from there